Method and apparatus for light internal reforming in a solid oxide fuel cell system

ABSTRACT

In an SOFC stack system, wherein a CPOx reformer supplies reformate to the stack, a portion of the anode tail gas is recycled directly into the anode inlet of the stack, such that the fuel reaching the anodes is a mixture of fresh reformate and recycled anode tail gas and is present at a sufficiently high temperature that endothermic reforming of residual hydrocarbons from the CPOx reformer occurs within the stack. Preferably, an amount of secondary non-reformed fuel is also added to optimize the fuel mixture presented for internal reforming. The anode tail gas is hot, at the stack temperature of 750-800° C., which allows for the mixture of anode tail gas and secondary fuel to be mixed and reacted in a clean-up catalyst to react higher hydrocarbons in the secondary fuel, without additional oxygen, prior to being mixed with reformate and sent to the stack.

This invention was made with United States Government support underGovernment Contract/Purchase Order No. DE-FC2602NT41246. The Governmenthas certain rights in this invention.

TECHNICAL FIELD

The present invention relates to solid oxide fuel cell (SOFC) systems;more particularly, to such systems wherein a portion of the anode tailgas from an SOFC stack is re-used by recycle; and most particularly, toa system wherein a portion of the anode tail gas is recirculateddirectly into the fuel cell stack.

BACKGROUND OF THE INVENTION

SOFC stack systems are well known. An SOFC typically is fueled by“reformate” gas, which is the partially oxidized effluent from acatalytic partial oxidation (CPOx) hydrocarbon reformer. Reformatetypically includes amounts of carbon monoxide (CO) as fuel in additionto molecular hydrogen (H₂). The CPOx reactions also release heat thatserves to maintain the temperature of the reformer. A CPOx reformer is avery simple and easily controlled device with good transient behaviorand dynamic range. A known disadvantage of a CPOx reformer is that ithas a relatively low fuel processing efficiency, generally in the rangeof 70-90%, that results in reduced overall system efficiency.

To improve stack power density and system efficiency and to reducecarbon precipitation and deposition in the system, it is known in theart to recycle a portion of the tail gas from the stack anodes into theinlet to the reformer. The stack anode tail gas has a large amount ofwater vapor and CO₂ as well as unreacted H₂ and CO gases. When thesegases are fed back to the reformer, endothermic “steam reforming”reactions occur in the fuel reformer in the well-known “water gas”reaction. Stack anode tail gas recycle is known to be enhanced by fuelreformer technology that can sustain its temperature in the presence ofendothermic reactions. Such technology may consist of a heat exchangerconstruction whereby hot combustor effluent passes on one side of theheat exchanger (combustor side), and fuel, air, and recycle gas mixpasses through the other side (reforming side). The reforming side iscatalytically treated to allow for the preferred reactions to occur.This mechanization yields high fuel processing efficiencies that, inturn, yield high system efficiencies.

Disadvantages to this approach are the complexity and potentialdurability issues with the heat exchanger/reformer device because of thehigher temperatures required for endothermic reforming; the systemcomplexity required to channel the combustor gases through the reformer;and the potential for carbon precipitation in the produced reformatewhich has low water vapor content by volume.

Where natural gas fuel is used, steam reforming with added water (norecycle) is a very common approach. In some cases, the natural gas fuelis pre-reformed to break-down higher hydrocarbons (above methane) andthis high-methane mix is fed directly to an SOFC stack. H₂O is typicallyadded to the reformate to allow steam reforming reactions to occurwithin the SOFC stack itself. This arrangement is known as “InternalReforming” in the art. In this approach, the heat to required forendothermic reforming to occur is supplied by the electrochemical heatreleased in the SOFC stack, and not by heat exchange with the combustorgases. Internal endothermic reforming within the SOFC stack is veryattractive for its high fuel processing efficiencies, but in the priorart it requires a supply of external water injection to the system.

What is needed in the art is a system for internal reforming thatrequires no external source of water, achieves high fuel processingefficiency, and permits simple construction and operation of the fuelreformer.

It is a principal object of the present invention to improve the fuelefficiency of a solid oxide fuel cell system.

SUMMARY OF THE INVENTION

Briefly described, in an SOFC stack system in accordance with theinvention, a conventional CPOx reformer supplies reformate to the stack.A portion of the resulting anode tail gas, which is rich in H₂, CO₂, andH₂O, is recycled directly into the anode inlet of the stack. Thus thefuel reaching the anodes is a mixture of fresh reformate and recycledanode tail gas and is present at a sufficiently high temperature thatendothermic reforming of residual hydrocarbons from the CPOx reformeroccurs within the stack. In a currently preferred embodiment, an amountof secondary non-reformed fuel is also added to the recycled anode tailgas to optimize the fuel mixture presented for internal reforming.

An advantage of this arrangement is that the anode tail gas has thehighest oxygen/carbon ratio anywhere in the system and also has a highconcentration of H₂O and CO₂. Therefore, the anode tail gas has theleast tendency to have carbon precipitation with the injection ofsecondary fuel.

A further advantage is that the anode tail gas is hot, nominally at thestack 20 operating temperature of about 750-800° C. This allows for themixture of anode tail gas, containing water vapor and CO₂, and secondaryfuel to be mixed and reacted without additional oxygen in a clean-upcatalyst prior to being mixed with reformate, to react longer chainhydrocarbons in the secondary fuel.

A still further advantage is that the resulting mixture of gases may berapidly cooled and fed to a recycle pump, thereby reducing the thermaldemand on such a pump. Cooling and pumping the pre-reformed mixtureprovides an extra degree of good mixing of the anode tail gas andsecondary fuel without further reactions occurring before entering theanodes.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described, by way of example, withreference to the accompanying drawings, in which:

FIG. 1 is a schematic flow diagram of a prior art SOFC system withoutrecycle of anode tail gas;

FIG. 2 is a schematic flow diagram of a prior art SOFC having recycle ofanode tail gas into the fuel stream ahead of the reformer;

FIG. 3 is a schematic flow diagram of an SOFC system in accordance withthe invention, showing recycle of anode tail gas into the inlet to theSOFC stack; and

FIG. 4 is a detailed flow diagram of the recycle and mixing portion ofthe diagram shown in FIG. 3.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, a first prior art SOFC system 10 comprises an SOFCstack 12 having an anode inlet 14 for reformate 16 from a CPOx reformer18; an anode tail gas outlet 20; an inlet 22 for heated cathode air 24from a cathode air heat exchanger 26; and a cathode air outlet 28. Anodetail gas 30 and spent cathode air 32 are fed to a burner 34, the hotexhaust 35 from which is passed through heat exchanger 26 to heat theincoming cathode air 36. The residual potential chemical energy (H₂ andCO) in the anode tail gas is not recovered as additional electricaloutput 38 of the stack but instead is partially recovered as heat energyin a heat exchanger 26.

Referring to FIG. 2, a second prior art SOFC system 110 comprises theelements just described for first prior art system 10. However, inaddition a first portion 140 of anode tail gas 30 is diverted ahead ofburner 34 to an anode tail gas cooler 142 and thence through an anodetail gas pump 144 which directs cooled portion 141 into an entrance toan air/fuel preparation chamber 146 ahead of CPOx reformer 18. Secondportion 143 of anode tail gas 30 is sent to burner 34 as in embodiment10, and the hot effluent 135 is sent to cathode air heat exchanger 26via a prior heat exchanger in reformer 118. Fortified reformate 116 issent to stack anode inlet 14. Thus, residual hydrogen and carbonmonoxide in the anode tail gas are transmitted to the stack for a secondtime, and heat is recovered in both the reformer and the cathode airheater. A heat source is critical to maintain the elevated temperaturesin the reformer during endothermic reforming. System 110 is known toimprove significantly the fuel processing efficiency of an SOFC system,resulting in an increase in electrical output 138, or a decrease in theamount of fuel needed to provide a fixed power output. However, as notedabove, significant practical problems are known in operating system 110,including a tendency for coking of the reformer, the increasedcomplexity of the fuel reformer/heat exchanger, and substantial thermalstresses on the reformer because of the elevated temperatures.

Referring to FIGS. 3 and 4, the arrangement of SOFC system 210, improvedin accordance with the invention, is substantially the same as that ofprior art embodiment 110 except for the following: Anode tail gas outlet241 from pump 244 is directed via pump 244 to the anode inlet 14 ofstack 12, bypassing reformer 18, where the anode tail gas joins withreformate 16 from reformer 18 to form a feed stream 216. The burnereffluent 235 bypasses reformer 18, as the undesirably high reformingtemperatures required in embodiment 110 are no longer necessary. Inaddition to the primary, independently controlled fuel flow 169supplying fuel 170 to reformer 18, a secondary, independently controlledfuel flow 269 is provided for supplying secondary fuel 270 into anodetail gas portion 240 to optimize the fuel stream 216 provided to stackanode inlet 12. Preferably, the tail gas/secondary fuel mixture ispassed through a clean-up catalyst 280 to reduce longer chainhydrocarbons to methane, H₂, and CO.

Primary fuel reformer 18, which is a simple and robust CPOx technologyreformer, supplies between 0% and 100% of the reformate to the SOFCstack, with typical values between 30% and 70%. At 100%, there is nosecondary fuel flow 270 to the recycle stream 216 and no internalreforming in the stack (0% internal reforming). At 0%, there is no CPOxreformate 16 to the stack and all of the fuel 241 is internally reformed(100% internal reforming). Both 0% and 100% cases are known in the art,but the subject of this invention allows for a mixture of CPOx and StackInternal Reforming strategies (between 0% and 100% internal reforming).This blended strategy, referred to herein as “Light Internal Reforming”,generally results in a reformate stream 216 to the stack that has a highconcentration of H₂ and H₂O, as well as moderate amounts of CO and CO₂,and a small amount (usually under 12%) of methane gas (CH₄). Thisarrangement allows for endothermic reforming within the stack itself forhigh fuel processing efficiencies and high electric output 238. Furtherthis arrangement allows for reduced internal reforming load (<100%) onthe stack which can improve durability. In addition, the CPOx reformerprimary fuel processing serves the needs of the system during thestart-up phase when the stacks are not operational but are warming-up,as well as under transient conditions where less internal reforming maybe desirable.

Technical Governing Equations:

Referring to FIG. 4, Omega (Ω) is the oxygen/carbon (O:C) ratio of thegas mixture at the described point. The O:C ratio is given byequation 1. $\begin{matrix}{\Omega = \frac{{moles}\quad O}{{moles}\quad C}} & {{Eq}.\quad(1)}\end{matrix}$The Primary Fuel Fraction, Psi (ψ), is given by equation 2. This is theportion of the total system fueling being reformed by the CPOx reformer.Note that 1-ψ is the portion of the total system fueling reformedinternally of the SOFC stack. $\begin{matrix}{\psi_{1} = {\frac{{\overset{.}{m}}_{{fuel},1}}{{\overset{.}{m}}_{{fuel},{total}}} = \frac{{\overset{.}{m}}_{{fuel},1}}{{\overset{.}{m}}_{{fuel},1} + {\overset{.}{m}}_{{fuel},2}}}} & {{Eq}.\quad(2)}\end{matrix}$The Primary Fuel Fraction may be expressed as a function of system fuelutilization [U], recycle fraction [r], Ω₁, and Ω₂, and the fuel H:Cratio [h2c] in equation 3. This equation is derived from the molarbalance in FIG. 4. $\begin{matrix}{{{\psi_{1} = \left( \frac{\Omega_{2} - {{U\left( \frac{r}{1 - r} \right)}\left( {2 - \Omega_{2} + \frac{h\quad 2c}{2}} \right)}}{\Omega_{1}} \right)};{\Omega_{1}<=\Omega_{2}}},} & {{Eq}.\quad(3)}\end{matrix}$Also from mole balance, the total fueling of the system can be describedin terms of stack current [I_(stack)], number of stack cells in series[N_(cells)], and the fuel properties including molecular weight (MW,g/mole) and stoichiometric coefficient of carbon (x_(fuel), whereC_(x)H_(y) is the fuel molecule). $\begin{matrix}{{\overset{.}{m}}_{{fuel},{total}} = {\left( \frac{I_{stack} \times N_{{cells},{stacks}}}{192970618} \right)\left( \frac{{MW}_{fuel}}{{U\left( x_{fuel} \right)}\left( {1 + \frac{r}{1 - r}} \right)\left( {2 - \Omega_{2} + \frac{h\quad 2c}{2}} \right)} \right)}} & {{Eq}.\quad(4)}\end{matrix}$From equations 3 and 4, the primary and secondary fuel rates may becomputed as:{dot over (m)}_(fuel,1)=ψ₁{dot over (m)}_(fuel,total)  Eq. (5)and:{dot over (m)}_(fuel,2)={dot over (m)}₁(1-ψ₁)  Eq. (6)

ψ₁ ranges between 0 and 1 with 0 corresponding to the case of 100%internal reforming and 1 corresponding to the case of CPOx reformingwith stack recycle (no internal reforming). For a given target Ω₁ andΩ₂, and a given stack fuel utilization, U, the limiting recyclefractions describing the limits of ψ₁ are given by equations 7 and 8.$\begin{matrix}{{r_{\min} = \left( \frac{\left( {\Omega_{2} - \Omega_{1}} \right)}{{U\left( {2 - \Omega_{2} + \frac{h\quad 2c}{2}} \right)} + \Omega_{2} - \Omega_{1}} \right)};{\psi = 1}} & {{Eq}.\quad(7)} \\{{r_{\max} = \left( \frac{\left( \Omega_{2} \right)}{{U\left( {2 - \Omega_{2} + \frac{h\quad 2c}{2}} \right)} + \Omega_{2}} \right)};{\psi = 0}} & {{Eq}.\quad(8)}\end{matrix}$To compute the effective reformer (fuel processing efficiency) of thesystem, the constants k and F in equations 9 and 10 may be computed, andthe efficiency computed in equation 11. $\begin{matrix}{k = \frac{\left( {2 - \Omega_{2} + \frac{h\quad 2c}{2}} \right)}{\left( {1 + \frac{h\quad 2c}{2}} \right)}} & {{Eq}.\quad(9)} \\{F = {{kx}_{fuel}\left( \frac{{\frac{1}{2}h\quad 2{c\left( {LHV}_{{H\quad 2},{mole}} \right)}} + {LHV}_{{CO},{mole}}}{{LHV}_{{fuel},{mole}}} \right)}} & {{Eq}.\quad(10)} \\{\eta_{reformer} = {F\left( {1 + \frac{r}{1 - r}} \right)}} & {{Eq}.\quad(11)}\end{matrix}$For completeness describing FIG. 4, the air flow 280 to the primary CPOxreformer 18 may be computed from equation 12. $\begin{matrix}{\left( {A/F} \right)_{reformer} = {\left( \frac{{\overset{.}{m}}_{air}}{{\overset{.}{m}}_{{fuel},1}} \right) = {\left( \frac{x_{fuel}{MW}_{air}}{{MW}_{fuel}} \right)\left( \frac{{\Omega_{1}\left( {1 + {U\left( \frac{r}{1 - r} \right)}} \right)} - {{U\left( \frac{r}{1 - r} \right)}\left( {2\frac{h\quad 2c}{2}} \right)}}{0.42} \right)}}} & {{Eq}.\quad(12)}\end{matrix}$

The benefits from Light Internal Reforming in accordance with theinvention may be demonstrated numerically for a system with, forexample, 60% fuel utilization, and Ω₁ of 1.25 in Table I. Note that highrecycle fractions and lower Ω₂ values tend to improve reformingefficiency. Reforming efficiencies described in Table I may be over100%, and may be as high as 159%, because it is calculated as the LowerHeating Value (LHV) of the reformate to the Stacks divided by the LHV ofthe fuel input to the system. Since the reformate to the stacks containsrecycled gases containing fuels, the reforming efficiency can exceed100%. TABLE I Effective Reforming (Fuel Processing) Efficiencies for LIREstimated Effective Reforming Efficiency O2C2 Recycle 1.5 1.55 1.6 1.651.7 1.75 1.8  0%  80%  78%  76%  75% 73% 72% 70% 10%  88%  87%  85%  83%81% 80% 78% 15%  94%  92%  90%  88% 86% 84% 82% 20% 100%  98%  96%  94%92% 90% 88% 25% 106% 104% 102% 100% 98% 96% 93% 30% 114% 112% 109% 107%105%  102%  100%  35% 123% 120% 118% 115% 113%  110%  108%  40% 133%130% 127% 125% 122%  119%  117%  45% 145% 142% 139% 136% 133%  130% 127%  50% 159% 156% 153% 150% 147%  143%  140% 

Table II describes the values for ψ₁ and the limiting recycle cases forthe reforming efficiencies tabulated in Table I. Note that calculatedvalues for ψ₁ that are above 1 or below 0 are outside the limit recyclefractions. Under these conditions, the system is either operating infull internal reforming domain (ψ₁<=0, or zero internal reforming domain(ψ₁>=1) and Ω₂ is not held to the target in the table (violatesequations). Under most cases of practical interest to the system, thetargeted values for ψ₁ are between 0.3 and 0.7. TABLE II LIR PrimaryFuel Split Fraction and Limiting Recycle Fractions O2C1 1.25 Utilization60% O2C2 Recycle 1.5 1.55 1.6 1.65 1.7 1.75 1.8 Formula for psi1 (2)Fuel split fraction (psi)  0% 1.20 1.24 1.28 1.32 1.36 1.40 1.44 10%1.07 1.11 1.15 1.19 1.24 1.28 1.32 15% 0.99 1.03 1.08 1.12 1.17 1.211.25 20% 0.90 0.95 0.99 1.04 1.08 1.13 1.18 25% 0.80 0.85 0.90 0.94 0.991.04 1.09 30% 0.69 0.74 0.79 0.84 0.89 0.94 0.99 35% 0.55 0.61 0.66 0.710.77 0.82 0.87 40% 0.40 0.46 0.51 0.57 0.62 0.68 0.74 45% 0.22 0.28 0.340.40 0.46 0.516 0.58 50% 0.00 0.06 0.13 0.19 0.26 0.32 0.38 Formula forrmin recycle minimum for psi1 <= 1 14% 17% 20% 22% 25% 27% 29% Formulafor rmax Formula for rmin recycle maximum for psi1 >= 0 50% 51% 53% 54%55% 56% 58%

While the invention has been described by reference to various specificembodiments, it should be understood that numerous changes may be madewithin the spirit and scope of the inventive concepts described.Accordingly, it is intended that the invention not be limited to thedescribed embodiments, but will have full scope defined by the languageof the following claims.

1. A method for operating a solid oxide fuel cell assembly including ahydrocarbon reformer, a fuel cell stack, a stack anode inlet, and astack anode outlet, comprising the steps of: a) providing reformate fuelfrom said hydrocarbon reformer; b) diverting a portion of anode tail gasfrom said stack anode outlet; c) combining said reformate fuel with saidportion of anode tail gas to form a combined fuel; and d) supplying saidcombined fuel to said stack anode inlet.
 2. A method in accordance withclaim 1 comprising the further step of is adding a hydrocarbon fuel tosaid portion of said anode tail gas to form a mixture thereof.
 3. Amethod in accordance with claim 2 wherein said adding step is performedbefore said combining step.
 4. A method in accordance with claim 2wherein said adding step is performed after said combining step.
 5. Amethod in accordance with claim 3 comprising the further step ofreforming hydrocarbons in said combined fuel within said stack.
 6. Amethod in accordance with claim 5 wherein said reforming is endothermic.7. A method in accordance with claim 5 wherein said reformate fuel fromsaid hydrocarbon reformer defines a first fuel source for said stack,and wherein said hydrocarbon fuel added to said anode tail gas defines asecond fuel source for said stack.
 8. A method in accordance with claim7 wherein said combined fuel comprises between about 100% and about 0%said second fuel source and between about 0% and about 100% said firstfuel source.
 9. A method in accordance with claim 8 wherein saidcombined fuel comprises between about 70% and about 30% said second fuelsource and between about 30% and about 70% said first fuel source.
 10. Amethod for operating a solid oxide fuel cell assembly including a fuelcell stack, a stack anode inlet, a stack anode outlet, and a stream ofanode tail gas emanating from the stack anode outlet, comprising thesteps of: a) diverting a portion of said stream of anode tail gas; b)adding a hydrocarbon fuel to said diverted portion of said stream; c)directing said diverted portion and said hydrocarbon fuel into saidstack anode inlet; and d) endothermically reforming said hydrocarbonfuel within said fuel cell stack.
 11. A method in accordance with claim10 comprising the further step of adding hydrocarbon reformate to saiddiverted portion of said stream prior to said directing step.